Known from WO 2006/108451 A1 is, for example, a speed-controlled force-limiting device for a seat belt retractor which has a plurality of masses disposed on the frame of the belt retractor that are made to oscillate. The masses are pivotably supported on the retractor frame and engage alternately, during the force-limiting retracting of the seat belt, with two teeth in an annular gear connected to the belt shaft. On the whole, due to the oscillating masses disposed on the retractor frame, the belt retractor is complex in production and expensive. In addition, due to the oscillating masses the belt retractor requires a relatively large installation space.
German patent application DE 10 2008 049 931.5 describes an improved force-limiting device operating on the same principle, wherein two toothed parts are moved in a force-limited manner toward each other, and one of the parts here executes an undulating feed motion during which the toothed parts alternatingly engage and disengage. The oscillating masses are replaced in this embodiment by the part executing the undulating feed movement, so that the force-limiting device described here requires a substantially smaller installation space and has a substantially simpler design. In one exemplary embodiment the part executing the undulating feed movement is realized by a toothed disc which is connectable in a friction-fit manner to the belt shaft. Between the toothed disc and the part against which the toothed disc executes the feed movement, a spring washer can be provided which exerts an axial contact force on the toothed disc, whereby the force-limiting level of the force-limiting characteristic is increased.
The force-limiting level of the force-limiting device results automatically from the to-be-diminished kinetic energy of the occupant during forward displacement, wherein the basic characteristic of the force-limiting path is strictly determined by the parameters of the toothed disc, such as the number of teeth, the separation of the teeth, the mass of the toothed disc and by the spring properties of the spring washer.
There are legal requirements, however, such as the US requirements for restraining forces for passengers in the back seats, which mandate an increase of the restraining forces to a higher force-limiting level after a preset length of seat-belt extraction. Such a force-limiting characteristic is not achievable with one of the above-described force-limiting devices.
In the patent application DE 10 2012 214 521, features are provided to resolve this problem using which the force required to move the parts toward one other can be automatically increased when a preset length of seat-belt extraction is reached. According to one embodiment of the proposed solution these means are formed by an open spring washer which applies a variable spring force to the parts moving toward one other during movement. In addition, during the movement the open ends of the spring washer come into abutment with the stop surfaces of the parts moving toward one another. Using the proposed solution, the spring washer is widened after the abutment of the ends on the stop surfaces, and the undulating shape of the spring washer is enhanced, so that as a result an increased spring force is applied to the two parts that are being moved toward one another, and the force-limiting level increases.
The object of the invention is to create a force-limiting device of the above-mentioned type which makes possible a progressive force-limiting course of the restraining forces, which can be constructed simply and has greater functional security.
In order to achieve this object, the invention proposes a force-limiting device having the features described herein by the Figures, and the accompanying description.